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Homework Help: Thevenin voltage help

  1. Dec 3, 2005 #1
    I'm trying to find the current and voltage across R2 using thenenin.
    http://img209.imageshack.us/img209/4606/thv15ph.png [Broken]
    I've managed to find Rth, which is equal to (R1 + R4)||(R3 + R5). That comes out to about 2.9 ohms. I need help with finding the Vth (Vab) though.
    http://img208.imageshack.us/img208/5002/thv28nc.png [Broken]
    Can someone please help me. What are the tricks involved here?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Dec 3, 2005 #2


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    Homework Helper

    Your second picture IS the trick ... with I_2 = 0, what is V_ab ?
    That is, with I_2 = 0 , all the current thru R_1 goes thru R_3 , R_4 , and R_5 .
  4. Dec 4, 2005 #3
    Ok then so with R2 removed, the circuit is now series. That makes the current the same for the remaining resistors.

    So if I_2 is 0, then the voltage between that point is also zero? That doesnt seem right though. I still dont get it, please give me another hint.
  5. Dec 6, 2005 #4
    Can no one help.

    Would I have to use the voltage divider? How do i use it?
    Last edited: Dec 6, 2005
  6. Dec 6, 2005 #5


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    Gold Member

    Yes you could use the voltage divider formula. With R2 removed, you can think of [tex]V_{AB}[/tex] as the voltage drops across [tex]R_3 and R_5[/tex]

    Look at it in this way. When you remove [tex]R_2[/tex], the circuit now becomes a series circuit. Look at where point A and B are. If you where to take a voltmeter and put the probes across points A and B. Your voltmeter would be giving you to combined voltage of R3 and R5.
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